Method of treating a gas mixture comprising propane and propylene

ABSTRACT

Methods for treating a gas mixture of at least propylene and propane, in order to separate the propylene from the propane. The gas mixture is brought into contact with a membrane enables the selective permeation of the propylene with respect to the propane. A propylene-enriched permeate and a propane-enriched retentate is formed. The propylene concentration of the permeate in the membrane is then reduced with a sweeping gas.

BACKGROUND

The present invention relates to a method for separating propane andpropylene contained in a gas mixture.

The propylene polymerization reaction for obtaining polypropylenegenerally uses:

-   -   propylene, as monomer,    -   a catalyst (Ziegler-Natta, metallocene, chromium oxide, etc.),    -   an agent controlling the length of the polymer chains, such as        hydrogen, and    -   inert substances issuing or not from the reaction (propane,        nitrogen, etc.).

During this polymerization reaction, unreacted propylene is usuallyrecycled to the inlet of the polymerization reactor. This recyclinginvolves a step of cooling or condensation of the effluent from thepolymerization reactor, in order to maintain a constant reactiontemperature. The propylene is recycled with other components of thereactor effluent such as hydrogen, propane and nitrogen. The propanerecycling is supplemented by a make-up of very rich and very purepropylene. Although this make-up is very rich in propylene, inertsubstances, such as propane and nitrogen, or light compounds, such ashydrogen, accumulate in the recycling effluent, thereby considerablyreducing the productivity of the polymerization unit. To decrease theconcentration of these inert substances or these light compounds, therecycling loop is occasionally purged of these substances. In otherwords, a part of the recycling loop is sent to a monomer recovery unitor flared. This purge operation has the drawback of decreasing theprofitability of the polymerization process because either an additionalunit is dedicated to propylene recovery, or the propylene is lost at theflare. To decrease these losses, it was proposed in patent U.S. Pat. No.B1-6,271,319 to treat the recycled effluent with a membrane permittingthe selective permeation of the propylene with respect to the propane.Thus, the membrane serves to obtain 1) a propane-enriched retentatewhich can thus be removed from the recycle loop and 2) apropylene-enriched permeate which can be recycled to the polymerizationreactor. The propylene losses are therefore limited in comparison withthe prior art and the productivity of the polymerization process isimproved. The drawback of this solution is that the membranes describedin U.S. Pat. No. B1-6,271,319 consist of materials selected frompolyimides, polyphenylene oxides and perfluoropolymers, which areslightly permeable to propylene. In consequence, for a given installedmembrane surface area, the propylene recovery rate remains low.

SUMMARY

The invention includes methods to achieve the desired results, asdescribed, but is not limited to the various embodiments disclosed.

The object of the present invention is to propose a method for improvingthe propylene/propane separation by permeation particularly in propylenepolymerization units.

A further object is to propose a method for improving thepropylene/propane separation by permeation in the propylenepolymerization units in order to increase the productivity ofpolypropylene production.

For this purpose, the invention relates to a method for treating a gasmixture comprising at least propylene and propane in order to separatethe propylene from the propane, in which the gas mixture is contactedwith a membrane (M1) carrying out the selective permeation of propylenewith respect to propane so as to obtain a propylene-enriched permeateand a propane-enriched retentate, characterized in that the propyleneconcentration of the permeate in the membrane is decreased by means of asweeping gas.

The invention further relates to a polypropylene polymerization method,comprising the following steps:

-   -   a) polymerization of propylene,    -   b) recovery of an effluent issuing from step a) and comprising        at least polypropylene, propane and propylene,    -   c) treatment of the effluent from step b) in order to produce a        solid effluent comprising at least polypropylene and a gaseous        effluent comprising at least propane and propylene,    -   d) treatment of the gaseous effluent issuing from step b) or        originating from the propylene recovery section, in order to        separate the propylene from the propane, in which the gaseous        mixture is contacted with a membrane (M1) carrying out the        selective permeation of propylene with respect to propane in        order to obtain a propylene-enriched permeate and a        propane-enriched retentate, characterized in that the propylene        concentration of the permeate in the membrane is decreased by        means of a sweeping gas,    -   e) recycling of the propylene-enriched permeate issuing from the        membrane (M1) to a polymerization step.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates a schematic view of one embodiment of a methodaccording to the present invention; and

FIG. 2 illustrates a schematic view of a second embodiment of a methodaccording to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention includes a procedure for the propylene/propane separationby permeation, as described above.

Hence the invention relates first to a method for treating a gas mixturecomprising at least propylene and propane in order to separate thepropylene from the propane, in which the gas mixture is contacted with amembrane (M1) carrying out the selective permeation of propylene withrespect to propane in order to obtain a propylene-enriched permeate anda propane-enriched retentate, characterized in that the propyleneconcentration of the permeate in the membrane is decreased by means of asweeping gas. The method according to the invention relates to thetreatment of a gas mixture comprising at least propylene and propane.The proportions of these two compounds in the mixture may be variableand are generally between 5% by weight and 25% by weight for propane and75% by weight and 95% by weight for propylene. This gas mixture maycomprise other compounds such as hydrogen, nitrogen, in proportions inthe range of 10% and 15% by weight respectively. This gas mixtureissuing from the polymerization reaction or from the propylene recoveryunit normally has a pressure of between 10 bar and 45 bar. Theseparation of the propylene and the propane from the gas mixture isobtained by a permeation method, that is, contacting of the gas mixturewith a membrane (M1) carrying out the selective permeation of propylenewith respect to propane. To ensure this selectivity between the twocompounds, the membrane (M1) preferably consists of a material selectedfrom polyimides and perfluoropolymers. The propylene preferably passesthrough the membrane and is recovered on the low pressure side formingthe propylene-enriched permeate, that is, which has a higher propyleneconcentration than the gas mixture to be treated. The propane preferablyremains on the high pressure side of the membrane and forms thepropane-enriched retentate, that is, which has a higher propaneconcentration than the gas mixture to be treated. According to theessential characteristic of the invention, the propylene concentrationof the permeate in the membrane is decreased by contacting a sweepinggas with the low pressure side of the membrane (permeate side). Thesweeping gas is normally introduced in countercurrent flow to the feedof gas mixture to be treated. The sweeping gas serves to increase thepropylene partial pressure gradient on either side of the membrane andaccordingly improves the productivity of the membrane. The sweeping gasdoes not contain any propylene and is preferably ethylene.

The method according to the invention can be implemented in particularduring a polypropylene polymerization process; in this case, thesweeping gas is advantageously a gas comprising ethylene (with a purity,for example, higher than 99% by weight), particularly if ethylene isused as comonomer during one of the polypropylene copolymerizationreactions. This particular implementation serves to improve thepropylene/propane separation efficiency while ensuring the premixing ofthe monomer, the propylene and the comonomer, upstream of thepolymerization reactor.

According to a particular embodiment of the method according to theinvention, when the gas mixture to be treated also comprises hydrogen,before the step of contacting the gas mixture with the membrane (M1)carrying out the selective permeation of propylene with respect topropane, the gas mixture may be contacted with a membrane (M2) carryingout the selective permeation of hydrogen with respect to propane and topropylene in order to obtain a hydrogen-enriched permeate and aretentate enriched with propylene and propane. This particularembodiment is recommended when the gas mixture to be treated compriseshydrogen in a non-negligible quantity, for example a hydrogenconcentration above 2% by weight. The membrane (M2) carrying out theselective permeation of the hydrogen normally consists of a materialselected from polyamides or polyimides. Preferably, the membrane (M2) isin the form of hollow fibers grouped in bundles forming a permeationmodule. According to a preferred application of this particularembodiment, the hydrogen concentration on the permeate side in themembrane (M2) carrying out the selective permeation of hydrogen isdecreased by means of a sweeping gas. The hydrogen concentration of thepermeate in the membrane (M2) is decreased by contacting the sweepinggas, preferably nitrogen or any other effluent containing no hydrogen,with the low pressure side of the membrane (M2) (permeate side). Thesweeping gas is normally introduced in countercurrent flow to the feedof the gas mixture to be treated. The sweeping gas serves to increasethe hydrogen partial pressure gradient on either side of the membrane(M2) and accordingly improves the productivity of the membrane. Thisparticular embodiment (combination of two membranes M1 and M2) serves toremove both the propane and the hydrogen from the gas mixture, whileminimizing the hydrogen losses.

The invention further relates to a propylene polymerization method,comprising the following steps:

-   -   a) polymerization of propylene,    -   b) recovery of an effluent issuing from step a) and comprising        at least polypropylene, propane and propylene,    -   c) treatment of the effluent from step b) in order to produce a        solid effluent comprising at least polypropylene and a gaseous        effluent comprising at least propane and propylene,    -   d) treatment of the gaseous effluent issuing from step b), or        originating from the propylene recovery unit, in order to        separate the propylene from the propane, in which the gaseous        mixture (1) is contacted with a membrane (M1) carrying out the        selective permeation of propylene with respect to propane in        order to obtain a propylene-enriched permeate (2) and a        propane-enriched retentate (3), characterized in that the        propylene concentration of the permeate in the membrane is        diluted by means of a sweeping gas,    -   e) introduction in the polymerization step a) of the        propylene-enriched permeate (3) issuing from the membrane (M1)        carrying out the selective permeation of propylene with respect        to propane.

According to this polymerization method, the polymerization reactoreffluent is generally treated during a step b) of cooling orcondensation. The gas phase comprises at least propylene and propane,and is then treated by the treatment method previously defined andpossibly by its preferred and particular embodiments.

Thus:

-   -   the membrane (M1) may consist of a material selected from        polyimides and perfluoropolymers.    -   the sweeping gas (4) may be a gas comprising ethylene or any        other effluent containing no propylene,    -   the gaseous effluent (1) issuing from step c) or issuing from        the propylene recovery unit may further comprise hydrogen and,        during the step of contacting the gaseous effluent (1) with the        membrane (M1) carrying out the selective permeation of propylene        with respect to propane, the gaseous effluent (1) may be        contacted with a membrane (M2) carrying out the selective        permeation of hydrogen with respect to propane and propylene, in        order to obtain a hydrogen-enriched permeate (5) and a retentate        (6) enriched with propylene and propane,    -   the membrane (M2) carrying out the selective permeation of        hydrogen may consist of a material selected from polyamides or        polyimides,    -   the hydrogen concentration of the permeate in the membrane (M2)        carrying out the selective permeation of hydrogen may be        decreased by means of a sweeping gas (7),    -   the sweeping gas (7) used during the selective permeation of        hydrogen may be a gas comprising nitrogen, or any other effluent        containing no hydrogen.

According to a particular embodiment, the step a) may be a polypropylenecopolymerization step. In this case, the sweeping gas (4) isadvantageously a gas comprising ethylene.

According to a further particular embodiment of the invention, theeffluent (1) issuing from step c) may be mixed with a gaseous effluent(1′) issuing from the successive sequence of a step a′) of polypropylenecopolymerization, followed by a step b′) of recovering the effluentissuing from step a′) and comprising at least polypropylene, propane andpropylene, followed by a step c′) of treatment of the effluent from stepb′) so as to produce a solid effluent comprising at least polypropyleneand the gaseous effluent (1′) comprising at least propane and propylene.This embodiment is adapted to the case of a propylene copolymerizationprocess. Thus, the membrane (M1) may treat a mixture of at least twogaseous effluents (1) and (1′) issuing, on the one hand, from thepropylene homopolymerization section, and on the other hand, from apropylene copolymerization section. The permeate (3) issuing from themembrane may then be recycled to the propylene copolymerization step.

The propane separated from the propylene is generally removed from theprocess and sent to the fuel gas network of the process or to the flare.

The propylene recovered on the permeate side of the membrane M1 isgenerally recompressed via existing compressors or via a new compressorbefore its introduction into the copolymerization section.

FIGS. 1 and 2 show the putting into practice of the method according tothe invention and more particularly the functioning of the membranes.FIG. 1 shows the functioning of the membrane (M1) carrying out theselective permeation of propylene with respect to propane. The gasmixture (1) comprising at least propylene and propane, and possiblyhydrogen and/or nitrogen, is introduced on the higher pressure side ofthe membrane (M1). A sweeping gas (4) having a lower pressure than thegas mixture (1) is introduced in countercurrent flow to the gas mixture(1) and on the permeate side of the membrane. The following compoundsissue from the membrane:

-   -   on the one hand, from the higher pressure side, the retentate        (2) enriched with propane compared with the gas mixture (1),    -   on the other hand, on the lower pressure side, the permeate (3)        enriched with propylene compared with the gas mixture (1).

FIG. 2 shows the functioning of the membranes (M1) and (M2) carrying outthe permeation of propane and hydrogen from the gas mixture comprisingpropylene, propane and hydrogen. The gas mixture (1) and/or (1′)comprising at least propylene, hydrogen and propane is introduced on thehigher pressure side of the membrane (M2). A sweeping gas (7) having alower pressure than the gas mixture (1) is introduced in countercurrentflow to the gas mixture (1) and on the permeate side of the membrane.The following compounds issue from the membrane (M2):

-   -   on the one hand, from the higher pressure side, the retentate        (6) enriched with propane and propylene compared with the gas        mixture (1),    -   on the other hand, from the lower pressure side, the permeate        (3) enriched with hydrogen compared with the gas mixture (1).

The retentate (6) of the membrane (M2) is then contacted with themembrane (M1) and in particular, is introduced on the higher pressureside of the membrane (M1). A sweeping gas (4) having a lower pressurethan the retentate (6) is introduced in countercurrent flow to theretentate (6) of the membrane (M2) and on the permeate side of themembrane (M1). The following compounds issue from the membrane (M1):

-   -   on the one hand, from the higher pressure side, the retentate        (2) enriched with propane compared with the gas mixture (1) and        with the retentate (6) of the membrane (M2),    -   on the other hand, from the lower pressure side, the permeate        (3) enriched with propylene compared with the gas mixture (1)        and with the retentate (6) of the membrane (M2).

It has been demonstrated that the removal of propane enhances catalystactivity and serves to increase the productivity of the propyleneproduction units. Thus, it has been possible to increase theproductivity of a Ziegler-Natta type catalyst by 0.5 to 2%,corresponding to additional quantities of polypropylene resin produced(about 2500 tonnes of resin for a polypropylene unit with a capacity of250 ktonnes/yr) while reducing the propylene losses.

The putting into practice of the method according to the invention alsohas the advantage of permitting the control of the propylene recoveryrate by controlling the flow rate of sweeping gas, contrary to themembrane methods of the prior art, in which the recovery rate dependedon the membrane surface area installed for a given permeate pressure.

The sweeping gas also serves to solve problems of membrane ageing(longer service life).

The particular embodiment of the method according to the invention hasthe advantage of improving the propylene/propane separation efficiencyby permeation while permitting separation of the hydrogen.

The use of an ethylene-rich sweeping gas or a propylene comonomer servesto improve the propylene/propane separation efficiency while ensuringthe premixing of the monomer (propylene) and ethylene or of the monomer(propylene) and the comonomer upstream of the copolymerization reactor.

EXAMPLES

With a polyimide membrane module having a membrane surface area of 470m², putting into practice the method according to the invention, it waspossible to recover 84% of the propylene present in the gas mixturecomprising 76 mol % of propylene and 24 mol % of propane. The sweepinggas used comprised 100% ethylene. The details of the characteristics ofthe application are given in Table 1 below.

TABLE 1 Propylene/propane separation by membrane with sweeping gas(ethylene) Gas introduced into the Sweeping membrane gas PermeateResidue Composition (mol %) hydrogen 0 0 0 0 ethylene 0 100 56.4 0.6propylene 76 0 40.2 39 propane 24 0 3.4 60.4 Partial molar flow (Nm³/h)hydrogen 0 0 0 0 ethylene 0 133 133.03 0.25 propylene 112.48 0 94.8517.55 propane 35.52 0 8.12 27.19 Molecular 42.6 28 34.2 43.2 weight(g/mol) Throughput 281.1 166.2 360.3 86.8 (kg/h) Throughput 148 133 23645 (Nm³/h) P(bar abs) 19 19 3.8 19 T (° C.) 148 148 90 90

By comparison, the permeation method carried out with the same membranebut without sweeping gas, according to the prior art technique, onlyserved to recover 71 mol % of the propylene present in the same gasmixture, as shown in Table 2 below.

TABLE 2 Propylene/propane separation by membrane without sweeping gasGas introduced into the Sweeping membrane gas Permeate ResidueComposition (mol %) hydrogen 0 0 0 0 ethylene 0 100 56.4 0.6 propylene76 0 40.2 39 propane 24 0 3.4 60.4 Partial molar flow (Nm³/h) hydrogen 00 0 0 ethylene 0 133 133.03 0.25 propylene 112.48 0 94.85 17.55 propane35.52 0 8.12 27.19 Molecular 42.6 28 34.2 43.2 weight (g/mol) Throughput281.1 166.2 360.3 86.8 (kg/h) Throughput 148 133 236 45 (Nm³/h) P(barabs) 19 19 3.8 19 T (° C.) 148 148 90 90

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A method which may be used to separate propylene from propane fromwithin a gas mixture during the polymerization of polypropylene, saidmethod comprising: a) contacting a gas mixture with a first membrane,wherein said gas mixture comprises propylene and propane; b) obtainingboth a propylene-enriched permeate and a propane-enriched retentatethrough the selective permeation, by said first membrane, of propylenewith respect to propane; and c) decreasing the propylene concentrationof said permeate in said first membrane with a first sweeping gas thatcomprises ethylene.
 2. The method of claim 1, wherein said membranecomprises at least one material selected from the group consisting of:a) polyimides; b) polyphenylene oxides; and c) polymers.
 3. A methodwhich may be used to separate propylene from propane from within a gasmixture, said method comprising: a) contacting a gas mixture comprisingpropylene, propane and hydrogen with a second membrane and obtaining apermeate enriched with hydrogen and a retentate enriched with propyleneand propane, by a selective permeation with said second membrane; b)contacting the retentate enriched with propylene and propane with afirst membrane and obtaining both a propylene-enriched permeate and apropane-enriched retentate through the selective permeation by saidfirst membrane, of propylene with respect to propane; and c) decreasingthe propylene concentration of said propylene-enriched permeate in saidfirst membrane with a first sweeping gas.
 4. The method of claim 3,wherein said second membrane comprises at least one material selectedfrom the group consisting of polyamides and polyimides.
 5. The method ofclaim 3, further comprising decreasing the hydrogen concentration ofsaid permeate enriched with hydrogen in said second membrane with asecond sweeping gas.
 6. The method of claim 5, wherein said secondsweeping gas comprises nitrogen.
 7. A method which may be used for thepolymerization of polypropylene, said method comprising: a) performing apolymerization of propylene; b) recovering an effluent from saidpolymerization, wherein said effluent comprises: 1) polypropylene; 2)propane; and 3) propylene; c) treating said effluent to produce a solideffluent and a gaseous effluent, wherein: 1) said solid effluentcomprises polypropylene; and 2) said gaseous effluent comprises propaneand propylene; d) treating at least part of said gaseous effluent toseparate said propylene from said propane, wherein said treating saidgaseous effluent comprises: 1) contacting at least part of said gaseouseffluent with a first membrane; 2) obtaining both a propylene-enrichedpermeate and a propane-enriched retentate, through a selectivepermeation of propylene with respect to propane, wherein said permeationis carried out by said first membrane; and 3) introducing a firstsweeping gas to decrease the propylene concentration of said permeate insaid first membrane; and e) introducing said propylene-enriched permeateto said polymerization of propylene.
 8. The method of claim 7, whereinsaid first membrane comprises at least one material selected from thegroup consisting of: a) polymides; b) polyphenylene oxides; and c)perfluoropolymers.
 9. A method which may be used for the polymerizationof polypropylene, said method comprising: a) performing a polymerizationof propylene; b) recovering an effluent from said polymerization,wherein said effluent comprises: 1) polypropylene; 2) propane; and 3)propylene; c) treating said effluent to produce a solid effluent and agaseous effluent, wherein: 1) said solid effluent comprisespolypropylene; and 2) said gaseous effluent comprises propane, propyleneand hydrogen; d) subjecting said gaseous effluent to a pretreatment,said pretreatment comprising: 1) contacting said gaseous effluent with asecond membrane; and 2) obtaining a permeate enriched with hydrogen anda retentate enriched with propylene and propane, by a selectivepermeation with said second membrane; e) treating at least part of saidretentate enriched with propylene and propane to separate said propylenefrom said propane, wherein said treating of said retentate enriched withpropylene and propane comprises: 1) contacting at least part of saidretentate with propylene and propane with a first membrane; 2) obtainingboth a propylene-enriched permeate and a propane-enriched retentate,through a selective permeation of propylene with respect to propane,wherein said permeation is carried out by said first membrane; and 3)introducing a first sweeping gas to decrease the propylene concentrationof said permeate in said first membrane; and f) introducing saidpropylene-enriched permeate to said polymerization of propylene.
 10. Themethod of claim 9, wherein said second membrane comprises at least onematerial selected from the group consisting of polyamides andpolyimides.
 11. The method of claim 9, further comprising decreasing thehydrogen concentration of said hydrogen enriched permeate in said secondmembrane with a second sweeping gas.
 12. The method of claim 11, whereinsaid second sweeping gas comprises nitrogen.
 13. The method of claim 7,wherein said polymerization of polypropylene comprises acopolymerization of polypropylene.
 14. The method of claim 13, whereinsaid first sweeping gas comprises ethylene.
 15. The method of claim 7,further comprising mixing said effluent with a second gaseous effluent,wherein said second gaseous effluent is produced by: a) performing acopolymerization of polypropylene followed by a recovery of a secondeffluent, wherein said second effluent comprises polypropylene, propane,and propylene; and b) treating said second effluent to produce a secondsolid effluent and said second gaseous effluent, wherein: 1) said secondsolid effluent comprises polypropylene; and 2) said second gaseouseffluent comprises propane and propylene.